Apparatus for distillation of gasoline containing hydrocarbon fractions



March 27, 195i P. scovlLLE APPARATUS FOR DI 2,546,349 sTILLATIoN oFGAsoLINE CONTAINING HYDROCARBON FRACTIONS 2 Sheets-Sheet l OriginalFiled Jan. l5

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March 27, 1951 P. scovlLLE APPARATUS FOR DIS 2,546,349 TILLATION 0FGAsoLINE CONTAINING HYDRocARBoN FRAcTIoNs Original Filed Jan. l5,

TORNEY QQ Inni Patented Mar. 27, 1951 l UNITED STATES PATENT OFFICEAPPARATUS FOR DISTILLATION OF GASO- INE CON TAINING HYDROCARBON FRAC-IONS 1 Claim. (Cl. ZOB- 153) l This invention relates to the manufactureof gasoline, such as aviation gasoline andmotor fuel, from normallygaseous and liquid hydrocarbons. More particularly, the inventionrelates 2 Still another object of the invention is to provide animproved method of stabilizing raw cracked naphtha and natural gasoline,and alkylating overhead 10W-boiling or normally gastothe stabilizationand fractionation of refinery 5 ecus isoparaifms with olens to produceaviation hydrocarbon fractions, such as cracked naphtha gasoline,wherein a greater recovery of low-boiland natural gasoline, and thecatalytic convering olefins and isoparahins is effected from the sionofeluent W-boiling or normally gaseous raw cracked naphtha and the naturalgasoline hydrocarbons to gasoline hydrocarbons of high to` thereby'materially increase the yield of aviaanti-knockvalue. 10 tion gasoline,While low-boiling normal parafin ThSS a-d-VSOH 0f my @pendingaDPH-Ca'GOIl v components are retained in the stabilized. naph- SeriallNo. 472,439, iiled January 15,I 1943, new tha or gasolinefor volatility,Patent N`0\2,.l42,440 dated TU-Ile 1,1948- A further object of theinventionI is to provide One of the. DriHCiDalObJ'eC'GS-Of theinventmliS apparatus for carrying out the 'method of this t0 DISOVde allimproved prOCeSS 0f stablzingjand 15 invention, which apparatus issimple in construcfractionating, petroleum products OI' flaCOnS, tion,is readily available andv easily controlled, Such as Cracked naphtha,and 0f Converting IOW- and' economical in operation andl maintenance.bllg or normally gaseous eiiluents of such sta- Other objects andadvantages of the invention bilization and fractionation to gasolinehydro- Wm. be apparent from the following description CaTbOIISOf highanti-RUOCK Value, t0 thereby 'll- 20 when taken inconjunction with theaccompanycrease' the yields of aviation gasoline and' motor ing drawingandV appended claim. fuel produced froml available refinery fractions,In the drawing Whichrmusl-,rates preferred em, elimina-te or minimizecorrosion', and secur'eother bodments 0f.1,he invention; advantages.Fig. 1 is a diagrammatic illustration of appa- Another object of theinvention is to provide a ral-,us for carrying out the method of thisinvennovel methodof stabilizing and fractonating non as applicato thestabilization of raw cracked hydrocarbon mixtures to obtain a betterseparanaphtha and the polymerization of el'uent gases tion of desiredfractions, while at the same time therefrom; and providing a' higherStabilization pressure and a Fig..2 is a diagrammatic illustration ofapparagreateroverhead condensateV recovery with availtus for carryingout the method of this invention able) cooling Water or otherv mediumfor the same as applied' to a modification involving the stabilireboilertemperature. zation andi fractionation of raw cracked naphtha Stillanother object of the invention is to` proand natural gasoline, and thealkylation of eiliuvide an improved method of stabilizingand frac'- entlow-boiling or normally gaseous isoparaflins tionating hydrocarbonmixtures containing both and olens therefrom. parains and olens of thesame molecular In conventional rerlnery practice', it is cus- Weight,whereby a portion of a normal paraffin tomary to stabilize' raw crackednaphtha as obmay be retained in the stabilized bottoms for tained" fromthe cracking of petroleum hydrovolatility While effecting a greaterrecovery of the carbons" to remove' normalv butane and lighter, oleflnsof the same molecular Weight in the over- 40 the eluent gases then beingtreated and conhead verted in part to gasoline hydrocarbons of highAnother object of the invention isA tok provide anti-knock value byvariousprocesses including an improved method of stabilizing raw crackedpolymerization and alkylation. The resulting naphtha and polymerizingoverhead low-boiling conversion products are stabilized and frac'- ornormally gaseous unsaturated hydrocarbons to tion'ated: to recover anaviation gasoline, and a polymer gasoline, wherein a portion of alowportion of the" normal butane may be blended boiling normal parafn isretained in the stabi'- back with the stabilized cracked naphtha forlized cracked naphtha for volatility, while at the volatility. Asapplied to polymerization, it is same time a greater proportion of theoleiins of customary' to stabilize the raw cracked naphtha the samemolecular weight as the low-boiling so to remove overhead C4 andlighter; The overnormal parafn are removed in the overhead to thereby:increasethe yield of polymer gasoline;

head gases may then be fractionated to separate a' selected C4'fraction, and the resulting C4' frac'- tion is then polymerized topolymer gasoline, stabilized and hydrogenated to provide a saturatedisoparanic aviation gasoline. On the other hand, the overhead gases maybe condensed, and the condensate comprising mainly Csi-C4 hydrocarbonspolymerized and stabilized to form a high octane motor gasoline. In aneffort to secure maximum yields of this gasoline, the raw crackednaphtha may be deeply stabilized to remove substantially all of thenormal butane and lighter, so that practically all of the C4 olens areobtained in the overhead. In this operation, difliculties may beencountered in condensing the desired amount of overhead gases with theuse of available cooling water without refrigeration due to the lowerstabilizer pressure required for a given available reboiler temperature;and the increased yield of polymerizable oler-ins in the stabilizeroverhead may be counter-balanced by the oleiins remaining uncondensedand passing to the renery gas lines, rather than being passed in thecondensate to the polymerization unit. On the other hand, if thestabilizer pressure is maintained constant, this deep stabilization ofthe cracked naphtha requires a higher reboiler temperature, which may beabove that normally obtainable with the available renery high pressuresteam supply. Also, the deep stabilization lowers the front endvolatility of the stabilized cracked naphtha so that it will not meetspecifications for motor uel, and necessitates the blending back oinormal butane with the stabilized cracked naphtha to meet front endvolatility speciiications.

Consequently, in many reneries, a compromise is effected on the basisor" the available high pressure steam supply and the available coolingwater temperature between the deep stabilization of the raw crackednaphtha described above and an operation in which a substantialproportion of the C4 hydrocarbons are left in the stabilized crackednaphtha for volatility. While this latter operation retainsnormaldoutane inV the stabilized cracked naphtha, it is also accompaniedby a Substantial loss of the C4 olens, particularly butylene-2, in thestabilizer bottoms. This results in a loss of desirable olens in thepolymer charge, with a resultant decrease in yield of aviation or motorgasoline from that theoretically possible from the available refinerystocks.

In order to operate at a suciently high stabilizer pressure to condensethe major portion of the desirable constituents of the overhead from theraw cracked naphtha stabilizer, it has been previously proposed toinjectwater into the reboiler of the stabilizer. However, this method isnecessarily limited to an increase in stabilizer pressure ofapproximately fifty pounds per square inch for a given reboilertemperature and, in'addition, may result in increased corrosion.

In accordance with the present invention, an improved method ofoperation is provided for stabilizing the raw cracked naphtha directlyto desired volatility and R. V. P., while at the same time the portionof C4 retained in the stabilizer bottoms consists largely of normalbutane, and a materially increased proportion of C4 oleiins for theparticular relatively low reboiler temperature used is secured in theoverhead. Moreover, ths method at the same time enables the stabilizeroperating pressure to be increased substantially more than theabove-noted fty pounds 4 for a given reboiler temperature without theuse of water injection, whereby an increased recovery of the valuableunsaturated hydrocarbons is obtained in the overhead condensate by theuse ofv available cooling water, or the same recovery can be obtainedwith a reduced overhead condenser surface area. This is accomplished byinjecting into the stabilizer an essentially saturated hydrocarbonfraction, such as a butane ,fraction which is heavier than the overheadfrom the stabilizer but is lighter than the average of the heavierstabilized fraction removed as bottoms. This injected hydrocarbonfraction may be obtained from an external source, but preferably isobtained as a product of the stabilization and fractionation of theresulting polymerization products from which unsaturated hydrocarbons orolefins have been essentially removed. The injected hydrocarbon fractionmay be introduced at any point within the tower below the point of entryof the normal reflux condensate, but preferably is added below the pointof entry of raw cracked naphtha charge. Very good results have beensecured by introducing this saturated C4 recycle stream into thestabilizer reboiler.

The invention is more particularly illustrated in Fig. 1, which showsthe raw cracked naphtha as obtained from a refinery cracking operationbeing charged by pump lil through line H into an intermediate point ofstabilizer l2. The latter may be a conventional fractionating tower ofthe multiple bubble plate type, and is equipped with a line I3 forwithdrawing liquid from a pool within the base of the tower and passingthe same through a reboiler I4 heated by high pressure steam supplied toheating coil l5, the heated hydrocarbon being returned to` the base ofthe tower by line i6. A bottoms of stabilized cracked naphtha iswithdrawn from the base of the tower by line Il. Overhead gases composedof C4 and lighter pass by line i8 to water-cooled condenser I9 andthence by rundown line 2!) to accumulator 2i, Uncondensed gas, mainly C2and lighter, is bled oiT from accumulator 2| by overhead line 22 tosuitable gas-collecting means not shown. Liquid condensate, consistinglargely of Cs-C4 is withdrawn from the accumulator through line 23 andforced by pump 24 through line 25. The latter communicates with branchedlines so that a portion of the condensate may be returned by line 2l tothe upper portion of tower i2 to serve as reiiux therein, while anotherportion passes by line 28 to a polymerization unit indicated generallyat 29.

Any conventional catalytic or thermal polymerization unit for convertingthe olenic con-- stituents of the charge to polymer may be ernployed.Since the polymerization unit per se forms no part of the presentinvention, further illustration thereof is deemed unnecessary. By way ofexample, the unit 29 may be a conventional phosphoric acid unit providedwith a heater in which the charge is raised to a temperature of aboutS-500 F. and then passed through a catalytic tower or convertercontaining kieselguhr on which is absorbed the phosphoric acid oatalyst.The operation thus far described is a socalled non-selectivepolymerization in which both C3 and C4 olefins are polymerized in thepresence of each other. l-Iowever, it is to be understood that theoverhead condensate from the stabilizer l2 could be further fractionatedto separate mainly a C4 fraction, whereby cross-polymerization of normaland isobutylenes occurs in the unit 29;. Infany event, it is; to beunderstood'. that the `(Iroleiins of the charge are largely consumed inthev polymerization step to. form higher boiling normally liquidpolymers.

The resulting polymerization products are passed by line 30 to adepropanizer 3|- where Cs and lighter are removed overhead by line. 32and the depropanized polymer gasolineY is withdrawn as bottoms by line33.. The overhead gas; passes to condenser 34 and thence byrundown line35.v to an accumulator 36 from which condensate is withdrawn by liney 3land returned by pump 38 through line 39 to an upper portion of tower 3I- to serve as reflux therein. It is also to be understood that thedepropanizer 3l may be equipped with a suitable heating coilor reboilerin the conventional manner.

The depropanized polymer gasoline passes by line 33 into a debutanizer40 also equipped with a conventional heating coil or reboiler at thebaseof the. tower (not shown), and from which a saturated C4 streamconsisting essentially of normal butane and isobutane is removed asoverhead by line: 4I and a stabilized polymer gasoline is withdrawn asbottoms by line 42 for passage to motor gasoline storage or to ahydrogenator (not shown) in the case. of selective polymerization in theproduction of aviation gasoline as is Well understood. The saturated C4gases pass by line 4I to condenser 43 and rundown line 44 to anaccumulator 45. The saturated C4 condensate is Withdrawn fromaccumulator 45 by line 46 and forced by pump 41 in part through line 48to the top of tower 4I! to serve as reilux therein, and in part by line49 to. the reboiler I4 of cracked naphtha stabilizer I 2.

Thel amount of saturated C4 which is recycled through line 49 tostabilizer I2 can be varied within substantial limits. For example, arelatively small amount of recycle on the basis of the raw crackednaphthal charged will give improved operation, and this can beA variedup to the recycle` of substantially all of the saturated C4 condensatenot required as reflux in debutanizer 40. Thus, a recycle rate of aslittle' as one part by volume of saturated C4 to a hundred parts byvolume ofraw cracked naphtha charged and up to equal parts by volume ormore may be employed. In the latter case, it is obvious that additionalsaturated C4 from an external source would be required in initiatingoperation and until there had been substantial buildup of the, saturatedC4 in the system. Generally, about one part of saturated C4 to twentyparts by volume of raw cracked naphtha charged up lto one part ofrecyclev to ten parts of naphtha charged are preferred to maintain theproper balance in the system'. It will be understood that this Will varyfor different reboiler temperatures and dilerent temperatures ofavailable cooling Water and can readily be determined in actualoperation by those skilled in this art for the particularconditionsencountered to provide the most enicient operating conditions.In operation, the amount of recycle may be controlled by anyconventional rate of flow controller in accordancer with the rate offeed of the raw cracked naphtha. to stabilizer I2. This is illustrateddiagrammatically by ratio flow controller 50 between line I I and line49 with air connection 5I regulating a valve 52 in recycle line 49. Itis to be understood that the hydrocarbon recycle can also be combinedwith direct. water injection into the stabilizer tower, if.desiredtoob.- tain the cumulative effecty on increase. in'tOWer.

pressurefor agiven reboiler temperature. of both '6 the. hydrocarboninjection and they water in- J'ection.

The following results were obtained in comparative runsv in commercialoperation on a combined raw cracked naphtha stabilization operation anda polymerization of the stabilizer overhead condensate by a conventionalphosphoric acid polymerization process. In one run, the raw crackednaphtha stabilizer was operated in conventional manner withouthydrocarbon recycle and injection into the reboiler of the stabilizer.In another run, a saturated C4 fraction obtained in the stabilization ofthe polymerization products as in Fig. l above was recycled at the rateof 14.6 b. p. h. into the reboiler of the raw cracked naphtha stabilizerto which the raw cracked naphtha was fed at the same rate as in theprevious run, namely, at about 208 b. p. h. The following were theconditions and results of the run:

Without Wth 14'6' b. p. h. -b-ut am butanc mjiton injected in o reboilerreboel.

Rates in barrels per hour:

Charge to raw cracked naphtha. 208 208 Butane injection into reboiler 014.6 Charge to polymerization unit 45. 55 60.98 Reflux to stabilizertower 124. 45 131.02 Reflux ratio 2.321 2. 0:1 Stabilizer bottoms, R. V.in 1bs 10. l l0. 5 Reflux material, Dist. I. B..P. F.. -44 -35 Refluxmaterial, Dist. 98%,011 at. p +30 +34 Pressure, lbs. per sq. in. gauge:-

Stabilizer Tower Top. 215 212 Redux accumulator drum 211 208 Highpressure steam for'reboiler- 174 172 Temperature, F.:

Stabilizer tower top 136 146 Rau/'naphtha to tower 208- 208 Reboilervapor 345 342 Reux accumulator drum.-. 88 88 Water supply to condenser8l 82 Water discharged from condenser 90 94 Charge plusbutane recyclecombined Liquid,

Vol. B. p. h Per Cent Liquid,

Vol. B. p. h. Per Cent Analysis Chg. to

Stabilizer:

Methane 0.42 0. 87 0. 39 0.87 Ethylene. 0. 73 l. 52 0. 68 1. 52 Ethauc3. 79 7. 88 3.54 7. 88 Propylene 3. 77 7. 84 3. 52 7. S4 Propane 6. 2613. 02 5. 85 13.02 Isobut'ane; 2. 30 4'. 78 3. 04 6. 76 Isobutylenc+butylene-l'. 3. 53 7. 34 63 8. 09 Normal butane 6. 5l 13. 54 10. 06 22.25 Buty1ene-2 3. 71 7. 72 4. 57 l0. 18 Pentaues+ (i8. 98 143. 49` 64. 78144. 19 Analysis, butano recycle:

Isobutane 13. 58 l. 08

5. 15 0. 75 59. 04 S. 7l 16. S4 2. 4G Pentanes+ .7e l o. 7o Analysisfeed to 2. 26 1.03 1.50 0. 91 4. 48 2. 04 2. 24 1.36 9. 54 4. 34 13. 908. 48 Propylene ll. 30 5. ,15 13. 32 8. 12 Propane 33. 34 15.19 28. 9217. 63 lsobutaue; 6. 35 2. 89 (i. 78 4. 13 lsobutylene+ butylene-l 8. 713. 97 il. 66 5. 89 Normal butano 15. 91 7. 25 15.79 fl. 63 Butylene2 6.7l 3. 05 (l. 49 3. 96 Pentmesnl. 40 O. 64 1. 40 0. 85 Analysisstabilized bottoms:

I s o b u t a n e -I- normal butano- 4. 33 6. 73 6. 96 11. 04 Butylenos3; 72 5. 78 2. 68' 4. 25 Pentanes+ 91. 95 142.85 90. 3G' 143. 34

Without Vltg l' .hmm butn 1n] ect1on im. ected mt-o into rcboileireboel.

Total C in stabilizer charge, b. p. h 33. 38 47. 28 Total saturatedbutane in stabilizer charge b. p. h 18. 32 29.01 Total unsaturatedbutylenes in stabilizer charge b. p. h 15.06 18. 27 Saturated butanes infeed to polymerization unit b. p. h 10.14 13.76 Unsaturated butylenes infeed to polymerization unit b. p. l1 7.02 9. S5 Saturated butanes tostabilizer bottoms b h 6. 73 11.04 to stabilizer bottoms b. p. h 5. 784. 25 Percentage of available saturated butanes Fecd to polymerizationunit 55. 35 47. 43 Stabilizer bottoms 36. 74 38. 06 Percentage ofavailable unsaturated butylencs to- Feed to polymerization unit 46. 6153.91 Stabilizer bottoms 38. 38 23. 26 Overhead gas from stabilizeraccumulator drum, b. p. h 7.14 2. 99

From the above, it will be noted that with substantially the same highpressure steam supply to the stabilizer reboiler and substantially thesame reboiler vapor temperature, and with approximately the same coolingwater temperature for the stabilizer overhead condenser, very materialadvantages were secured with the butane recycle and injection into thereboiler of the stabilizer. Thus, the percentage of available butylenesleft in the stabilizer bottoms dropped from approximately 38% to 213%,while the percentage of available butanes left in the bottoms wasincreased from approximately 37% to 38%, and the Reid vapor pressure ofthe stabilized bottoms was maintained or even slightly increased from10.1 to 10.5 pounds. Moreover, the percentage of available butylenes inthe feed to the polymerization unit was increased from approximately46.5% to 54% while the percentage of available saturated butanesdecreased from approximately 55.5% to 47.5%. At the same time, there wasa substantially increased condensation of the stabilizer overhead andconsequently an increase in the charge to the polymerization unit, as isshown by the drop in the overhead gas from the accumulator drum from'7.14 to 2.99 barrels per hour.

It is thus seen that, by the method of .the present invention, the rawcracked naphtha was stabilized to desired volatility and Reid vaporpressure while at the same time a substantially improved separationbetween saturated butanes and unsaturated butylenes was secured so thata greater proportion of the saturated butanes remained in the stabilizedcracked naphtha while a greater proportion of the unsaturated butyleneswas removed overhead, condensed and thus recovered for thepolymerization feed. These beneficial results were secured solely byrecycling the available butane fraction from the polymer debutanizer tothe stabilizer reboiler, and without vention has been described inconnection with 8 the stabilization of raw cracked naphtha to obtain aseparation between saturated butanes and unsaturated butylenes, it is tobe understood that the invention is also applicable to effecting adesired separation between pentanes and amylenes, hexanes and hexylenes,etc.

Referring to Fig. 2, the invention is shown as applied to thestabilization of raw cracked naphtha and natural gasoline and thealkylation of isobutane with C4 and C5 oleiins to thereby materiallyincrease the yields 0f aviation gasoline from available charge stocks.As shown, raw cracked naphtha from a conventional cracking unit isintroduced by line 60 to stabilizer 6I equipped with reboiler 62, andoperated to take overhead by line 63 a C5 and lighter fractioncontaining a substantial proportion of the C5 olens. A stabilizedcracked naphtha containing a substantial proportion of normal pentanefor volatility is withdrawn as bottoms by line 64. The overhead fractionis passed through condenser 65 to accumulator 66. Condensate is forcedby pump S1 in part through line 68 to serve as reflux in tower El, andin part by line B9 to a depropanizer 10. In the latter, the charge isfractionated to remove overhead through line 1| C3 and lighter, andobtain a bottoms cut consisting essentially of C4 and C5 which is passedby pump 12 through line 13 to a conventional alkylation unit 14.

While any conventional alkylation process can be employed, the inventionis described for purposes of illustration in connection with a sulfuricacid alkylation step. In this process, an additional supply of isobutaneintroduced by the recycle line 'i5 to be hereinafter further described,or from any other suitable source, is mixed with the olefinic Cil-C5charge from line 13 in the presence of strong sulfuric acid of about88-100% concentration at temperatures of about 35-'70o F. and undersufficient pressure to maintain the hydrocarbon in liquid phase. Anyconventional type of unit, such as the well known pump and time tankreactor, the jet reactor, or the mixer of the turbo type can be used.The isobutane is maintained in substantial molar excess of the olefins;

and preferably the operation is of the well known emulsion recycle typein which contact ratios of from 50:1 up to 200:1 or higher are employed.Under these conditions, the isobutane is alkylated by the C4 and C5oleflns to produce good yields of gasoline hydrocarbons or alkylate ofhigh anti-knock value. It is to be understood that other well knownalkylation catalysts, such as hydrofiuoric acid, aluminumchloride-hydrocarbon complex, BFS-water complex, and the like can beused; although strong sulfuric acid is preferred for this operation. Asthe alkylation unit per se forms no part of the present claimedinvention, further description thereof is unnecessary. It is understoodthat in conventional practice, a stream of the reaction products iswithdrawn to a suitable settler where the catalyst is separated from thehydrocarbons, and the latter are then neutralized before being passed tothe stabilizing and fractionating equipment. These various steps and theequipment therefor are well known, and it is to be understood that thediagrammatic illustration of the alkylation unit represented by thenumeral 14 includes the alkylation reactor, settler and neutralizer.

The neutralized hydrocarbon reaction products from the alkylation unitare passed by line 11 to a debutanizer 78 where unreacted C4hydrocarbons consisting essentially of excess isobutane and normalbutane are removed overhead by line 19 and a debutanized alkyl'ate iswithdrawn as bottoms by line 80. As the alkylation reaction consumes theolenns in the charge, the overhead C4 stream is further 'fractionated ina butane fractionator 89 where Aan isobutane out is removed overhead andrecycled by line 15 to the alkylaton unitV While a normal butane streamis removed as bottoms by line 8| Vand passed in whole or part by pump 82through line 83 to the stabilized cracked naphtha in line 64 to increasethe yield and volatility of the motor gasoline.

The debutanized alkylate withdrawn by line 84 is `passed by pump 85through a line 8S to a depentanizer 81 where C5 is removed overhead byline 88 and depentanized alkylate is removed as bottoms by line 89. Itis to be understood that the above-described order of towers 'can bechanged. For example, the raw alkylate can rst be depentanized, and theresulting C5 and lighter offgases fractionated in la series of towers toobtain the various fractions described, including an isobutane cut, anormal butane cut, an isopentane out and a normal p'entane cut. The C5overhead fraction consisting essentially of isopentane and normalpentane is mixed with a stream of debutanized natural gasolineintroduced by line 99, and the mixture passed vinto a de-isopentanizer9|. The latter is operated to remove isopentane overhead by line 92, andthe de-isopentanized natural gasoline containing the normal pentane fromthe alkylate is removed as bottoms by line 93. The latter is passed bypump 94 to a depentanizer 95 where normal pentane is removed roverheadby line 96 and the depentanized natural gasoline discharged as bottomsby line 91. It is to be understood that the fractionating towers 10, 18,89, 81 and 9| are equipped with the usual reboilers or other heatingmeans for the base of the towers, and with suitable condensers andaccumulators for the overhead and pumps for supplying condensate asreflux to the tops of the towers as well as forcing the remainingportion of the condensate to the next tower or unit in series, whichelements are not shown for simplicity in illustration.

The overhead normal pentane fraction passes by line 96 to condenser 99and thence by rundown line |99 to accumulator lill from which condensateis withdrawn through Yline |92 by pump |93 and forced in part throughreflux line |04 to the top of tower 95, and in part by recycle line |05to the reboiler 92 of the cracked naphtha stabilizer `[il In thismanner, a saturated C5 stream consisting largely of normal pentane isprovided for injection into stabilizer 6| to thereby obtain a morecomplete separation of C5 clef-ins in the overhead, while retaining adesired .proportion of normal pentane in the stabilized cracked naphthabottoms. While the above described introduction of debutanized naturalgasoline into the fractionating system is advantageous for theproduction of more isopentane, it is to be understood that this is notessential in accomplishing the broader objects of the present invention.For example, the C5 cut from the stabilization of the alkylate may bedirectly fractionated into isopentane and normal pentane fractions,without addition of any natural gasoline cut. l

The depentanized alkylate removed from tower 81 by line 89 is forced bypump 01 through line |08 to the alkylate fractionator |09 where adesired aviation gasoline fraction boiling up to about S50-375 F. isremoved overhead by line Il!) and alkylate bottoms are discharged byline and forced by pump H2 through line l3 to storage or for furthertreatment. Preferably, the alkylate bottoms are passed by line ||3 tosuitable blending tanks (not shown) where they are mixed with thestabilized cracked naphtha from line 64 to increase the yield of motorgasoline. The overhead aviation fraction is condensed in condenser l l 5and passed through rundown line ||6 to accumulator ||1 from which theaviation gasoline is discharged by line |8 to storage or for furthertreatment. The overhead isopentane fraction from line 92 is passed tosuitable condenser and accumulator equipment (not shown) and thecondensate blended with the aviation gasoline from line I8 to therebyincrease the yield and adjust the volatility of the aviation gasoline.It is to be understood that the depentanized natural gasoline from line91 may, if desired, be passed to a further fractionating tower (notshown) serving as a, de-isohexanizer from which an overhead isohexaneout may be obtained for blending with the aviation gasoline from linel|8 to further increase the yield thereof. The resulting naturalgasoline bottoms from this further fractionating operation may then bepassed to reforming or other treatment, or may be blended directly withthe stabilized cracked naphtha from line 94 for motor gasoline.

It is to be understood that the debutanized natural gasoline introducedbyline has been previously stabilized to separate C4s and lighter, andthe offgases from this stabilization fractionated to `separate anisobutane-rich fraction which is introduced by line |29 into theisobutane feed for the alkylation unit 14 to thereby serve as anexternal source of Ysupply of additional isobutane. A normal butane alsorecovered from the natural gasoline in this latter operation may beadded to the normal butane stream passed by line 83 to the stabilizedcracked naphtha in line B4. These elements are not illustrated in thedrawing for the sake of simplicity in illustration.

While the invention has been described above in combination with asuitable polymerization or alkylation unit, from which the desiredrecycle stream is obtained for injection into the cracked naphthastabilizer, it is to be understood that the invention is not limited tothese particular catalytic conversion units. Thus, it is obvious thatthe stabilization method described herein can be applied in conjunctionwith other catalytic or thermal conversion units, such as isomerization,cyclization and aromatization, and various combinations of these withpolymerization and alkylation. Likewise, the invention is not limited tothe stabilization of cracked naphtha and natural or straightrun gasolinefractions, since the particular stabilization and fractionation methodis more broadly applicable to the distillation of olefin and paraffincontaining hydrocarbon mixtures generally.

In addition, the stabilizing and fractionating method of the presentinvention can be used to eifect more complete separation between anoverhead isoparafiin and lighter out from a bottoms cut containingolefins of the same molecular Weight as the overhead isoparamn. In thiscase, the recycle stream or hydrocarbon stream injected into thestabilizer will be one which is enriched in the said olens to beretained in the bottoms and which is relatively lean in the isoparaffinto be removed overhead.

The invention can also be employed in a stabilizing operation where theoverhead condensate capacity is insufficient for normal operation, tothereby enable the stabilizer to operate at a higher pressure for thesame reboiler temperature, with resultant increased condensation of theoverhead at the higher pressure with the same condenser cooling surface.Likewise, the invention can lbe applied to stabilizing operations wheredifficulty is encountered due to the temperature of available coolingwater for the overhead condensate being too high, since the higherstabilizer pressures resulting from the hydrocarbon injection permitincreased overhead condensation with the higher temperature coolingwater.

Obviously many modifications and variations of the invention, ashereinbefore set forth, may be made without departing from the spiritand scope thereof, and therefore only such limitations should be made asare indicated in the appended claim.

I claim:

Apparatus of the character described, comprising in combination, afractionating tower of the multi-plate type, a liquid feed line openinginto an intermediate portion of said tower, a vapor offtake from theupper portion of said tower, a condenser and accumulator connected tosaid vapor offtake, a liquid discharge line from the lower portion ofsaid accumulator, a pump connected to said liquid discharge line and inturn connected to a return line opening into the upper portion of saidfractionating tower above said liquid feed line to supply reux to thetower, a rst liquid discharge line from the base of said fractionatingtower, a second liquid line discharging from a lower portion of saidtower above said first liquid discharge line, a vessel equipped with anindirect heating coil connected to said second liquid line to receiveliquid discharged thereby and to heat said liquid to a reboilingtemperature, a return line from an upper portion of said vessel andopening into a lower portion of said tower above said first liquiddischarge line to return heated liquid and vapor from said vessel tosaid tower, a liquid feed line for extraneous liquid opening into thelower portion of said vessel, a flow controller for said extraneousliquid feed line, and operative connections from said ow controller tosaid liquid feed line opening into said tower and to said extraneousliquid feed line opening into said vessel to thereby regulate the rateof feed of said extraneous liquid to said vessel in accordance with therate of feed through said liquid feed line to the tower.

LOREN P. SCOVILLE.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,006,186 Stines June 25, 19352,072,093 Blakey Mar. 2, 1937 2,168,316 Brandt Aug. 8, 1939 2,172,560Kemp Sept. 12, 1939 2,221,425 Ruthruff lNov. 12, 1940 2,286,504 ParkerJune 16, 1942 2,307,024 Carney Jan. 5, 1943 2,348,931 Schulze May 16,1944 FOREIGN PATENTS Number Country Date 492,567 Great Britain Sept. 22,1938

